How Long Does Weed Stay in Your System?

Commonly known as cannabis, weed, dope, grass, or pot, marijuana is a popular recreational drug known for its ability to induce calmness and euphoria. In the UK, marijuana is categorised as a Class B drug, making it illegal to produce, sell or use recreationally. For this reason, cannabis drug tests are commonplace in a variety of settings including the workplace, the criminal justice system, and sports.

It’s no surprise then that “How long does weed stay in your system?” is one of the most searched terms about marijuana on the internet. This article provides a detailed answer. We take a look at how the body processes the active ingredient in marijuana, the different testing methods available, and the factors influencing how long it remains in the body.

What Is the Active Ingredient in Weed?

Marijuana products can derive from three varieties of the cannabis plant – Cannabis sativa, Cannabis indica, and Cannabis ruderalis. The psychoactive effects of the drug largely come from a chemical compound, known as a cannabinoid, called delta-9-tetrahydrocannabinol (THC). THC is found in resin produced by the leaves and buds of the female cannabis plant. The ‘high’ that THC produces is a result of the way that it interacts with our brain’s reward system.

How Does Your Body Process THC and How Long Does Weed Stay In Your System?

How Does Your Body Process THC and How Long Does Weed Stay In Your System?

The route that TCH takes through the body depends on whether you inhale (smoke or vape) or ingest (eat) cannabis. 


When smoked or vaped, the THC in cannabis is absorbed by the lungs and rapidly circulated via plasma and red blood cells to the brain and other organs. This is why smoking or vaping weed produces an almost instant ‘high’. It is then absorbed into tissues including the brain, heart, and fat. The liver eventually metabolises the THC – a process that breaks THC down into byproducts (metabolites) that the body can use in its cellular processes. In the last stage of its journey, THC metabolites are eliminated from the body as waste products in urine and faeces. 


When cannabis is consumed as an ‘edible’, THC takes a slightly more complex route through the body before the user will feel its effects. The drug enters the digestive system and is absorbed through the intestines. It is then metabolised by the liver before entering the bloodstream and the brain. As with inhaled cannabis, the THC is then absorbed into tissues, metabolised by the liver a second time, and finally excreted from the body. 

Infographic explaining how long cannabis stays in your system.

How Is Marijuana Drug Tested?

A drug test is used to detect THC and/or its metabolites and other drugs in a person’s system. It involves running a technical analysis of a biological sample – usually urine, saliva, blood, or hair. The type of test used will depend on several factors including the level of accuracy required, detection times, cost-effectiveness, the personnel available, and how quickly the results are needed. It’s worth noting that false positives and, in some cases, false negatives can occur with most drug tests. This is why a second confirmatory test (Gas Chromatography-Mass Spectrometry or GC-MS) can be helpful depending on the situation. 

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Urine Testing for Cannabis

What Does It Involve?

A urine test is one of the most common forms of testing for cannabis. You will be asked to provide a fresh urine sample on-site, and the tester will perform the test in front of you. This involves measuring the temperature of the sample to ensure that it is within the normal range and then dipping the test stick into the sample. Depending on the type of test used, you may get the results immediately (rapid testing) or the sample may be sent away for lab analysis to provide greater accuracy or if a formal written report is required.

When Is It Used?

Urine testing is used in a variety of settings, including:

Healthcare – for example, if you are suspected of being under the influence of cannabis when you present at A & E.
Employment – as part of pre-employment checks and employee monitoring. 
Addiction treatment centres – routine and spot testing to monitor abstinence.
Prison, probation, and parole – to monitor compliance with the regime/licence conditions. 

What Is the Detection Time?

Depending on usage, cannabis can be detected up to 30 days after use. Average detection times are as follows:

One-off and infrequent usage –  detectable for 3 days after use
Moderate use (4 times a week) – detectable for 5-7 days 
Chronic use (daily) – detectable for 10-15 days 
Chronic heavy use (multiple times daily) – detectable for up to 30 days

Saliva Testing for Cannabis

  • What Does It Involve?

    Also known as mouth swab testing and oral fluids drug testing, a saliva test is quick, easy, and accurate. Depending on the type of test used, you may be told not to eat or drink anything for 10 minutes before the test. The collector will then ask you to open your mouth so that they can take a sample of saliva with a collection stick. The test is processed in front of you. Results may be immediate or the sample may need to be sent away for lab analysis.

  • When Is It Used?

    Saliva tests are generally used by:

    • Police – for roadside testing.
    • Employers – as part of pre-employment checks and employee monitoring.
    • Addiction treatment centres – routine and spot testing to monitor abstinence.
    • Prison, probation, and parole – to monitor compliance with the regime/licence conditions.

  • What Is the Detection Time?

    Most studies indicate that saliva tests can detect cannabis from 24-72 hours after last use in infrequent users. However, research by Lee and Huestis in 2013, evidenced that THC could be found in the saliva tests of chronic users up to 29 days after their last use. As with most rapid testing, false positives are a possibility.

Blood Tests for Cannabis

What Does It Involve?

Blood tests provide high accuracy but within a short detection time. Depending on the type of test being conducted, the collector will take either a finger-prick sample or a test tube sample of blood from a vein in your arm. It is not possible to obtain on-site results from a blood test as the sample has to be sent to a laboratory for analysis.

When Is It Used?

Blood drug testing is generally limited to testing at crime scenes, and accident sites to establish whether those involved were under the influence of cannabis at the time of the incident. This type of test is also sometimes used when people are applying for insurance or going through a job application process. 

What Is the Detection Time?

Hair Tests for Cannabis

What Does It Involve?

During a hair follicle test, the collector will cut around 100 hairs from the crown of your head. These can be taken from different spots to avoid creating a bald patch. If you have little or no hair on your head, the collector may use body hair instead. After collection, the sample is wrapped in foil and sent to the lab for testing and analysis. This type of testing can produce false positives if you have been exposed to second-hand smoke or if cannabis somehow comes into contact with your hair. 

When Is It Used?

Hair drug testing is ideally suited to situations where it is necessary to build a picture of the history of a person’s cannabis use over some time. For this reason, it is likely to be used by entities such as employers, courts, and social services. This type of testing may not pick up a single use of a drug or infrequent episodes of low use. 

What Is the Detection Time?

Depending on the type of test used and the length of the hair strands being tested, THC can be detected in the hair follicles after 7 days and for up to 12 months after the last use of cannabis. However, the average detection time is up to 90 days. 

How Long Does Synthetic Marijuana Stay In Your System?

Synthetic marijuana (commonly known as “Spice”, “K2” and “legal high”) contains many ingredients and no two batches are the same. This means that it can be very difficult to determine the amount of time that it takes for this type of drug to leave the body. Most synthetic marijuana can be detected in the blood and saliva for up to 72 hours, but this is extremely variable.

As synthetic cannabis is a fairly new category of drug, testing methods and research studies are still in their infancy. Standard drug tests do not currently test for synthetic cannabinoids, but specialist testing kits are available on the market. 

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Factors That Affect Detection Time

How long THC is detectable in the body depends on several varying factors, including the frequency of your drug use, the amount of fatty tissue in your body, how you use the drug, and whether you are male or female. 

Frequency of Use

Light and infrequent users most likely metabolise the THC through the liver quickly with little being absorbed into fatty tissue. Chronic users will have THC stored in their fat cells indefinitely while still using and for extended periods afterwards. The majority of frequent to heavy users report a positive test 45 days from their last use.

Body Mass Index (BMI)

Body Mass Index (BMI) is a measurement of a person’s body fat based on height and weight. Generally speaking, people with a higher distribution of body fat will store THC in their body (via fatty tissue) for longer than people with a lower BMI who have less capacity to retain THC.

Smoking vs. Vaping vs. Edibles

Smoking or vaping allows THC to enter the bloodstream rapidly via the lungs. This leads to an almost instant high and, similarly, faster processing by the liver. Edibles, on the other hand, are harder to absorb and the effects often linger in the body for longer. 


A trend toward higher levels of fat distribution and slower metabolism in females may result in the body ‘storing a higher level of THC in fatty tissue, making it detectable for longer. In addition, research shows that oestrogen levels may influence the Endocannabinoid system. This can have a knock-on effect on how females experience the effects of cannabis and process THC.  

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Can You Fail a Drug Test From Secondhand Smoke?

Although very little research has been conducted into the effects of secondhand smoke, it’s thought that minimal levels of THC are expelled in breath making failing a drug test unlikely. However, under extreme conditions, it is possible to show some detectable level of THC, often not enough to warrant a positive result. As mentioned previously, hair contaminated with secondhand smoke may produce a false positive in a hair follicle test.

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  1. What is the definition of Cannabis?
  2. Current Knowledge on Cannabinoids in Oral Fluid
  3. How Long Does Cannabis Stay in Your System?
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  5. Sex, drugs and estradiol: Why cannabis affects women differently

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